In a circular fluorescent lamp, mercury typically is provided to bulb, using a method of filling mercury in a bulb, a method of dropping and introducing liquid mercury into the bulb directly from an exhaust-pipe (vacuum-pipe) (hereinafter, "a dropper method" will be referred to). Since it is difficult to control the filling amount of mercury by the dropper method, a great amount of mercury has to be used so as to ensure the reliability of products. However, since mercury is a harmful substance for the environment, it is desirable to minimize the amount of mercury.
In order to reduce the amount of mercury, alternative methods to the dropper method have been considered. One such method includes filling only a required amount of mercury by providing a mercury-releasing metal substrate, for example, an alloy of zinc and mercury inside a bulb. However, in a case where the mercury-releasing metal substrate is merely put into the bulb, the mercury-releasing metal substrate moves freely inside the bulb. As a result, the mercury-releasing metal substrate makes noises or peels off an inner fluorescent film. To overcome this, the mercury-releasing metal substrate is adhered to the inside face of the bulb by bringing the mercury-releasing metal substrate into contact with a seal part of the end of the bulb and then heating the bulb from the outside.
As a method of heating the bulb from the outside, a method of heating by a furnace, burner, or the like, is employed. However, under the conventional heating conditions, the mercury-releasing metal substrate cannot be adhered or is insufficiently adhered to the bulb, so that the mercury-releasing metal substrate is often peeled off from the bulb.